In applications such as printed circuit board (PCB) production, a laser may be used to expose a pattern on a photoresist coating on a copper coated substrate. In a typical exposure system such as the DP 100 of LIS of Germany, a CW UV laser beam is scanned across the PCB surface, while its intensity is modulated in accordance with a pattern to be generated. The modulating device receives electronic pixel data supplied by control circuitry. In modern PCB production it is desirable to operate at high data rates to increase production speed. The practical data rate is limited by the modulation rate and/or the available laser power.
In the production of PCBs utilizing UV sensitive photoresists an Argon Ion laser is often used. Although they are widely utilized as UV light sources, Argon lasers, being gaseous lasers, have a number of drawbacks such as complicate and delicate to operate, poor maintainability and/or high price.
Methods for producing UV laser radiation are known. For example, one such method utilizes an IR mode locked laser to generate high repetition rate laser light pulses. The frequency of the laser light is doubled to UV by passing the light through a non linear medium. However, the utilization of such lasers to photoresist exposure is severely limited by an inherent paradox: practical UV power drops rapidly with an increase in the repetition rate. This is because the frequency doubling process is extremely non linear and thus, its efficiency increases with increasing peak power. For a given average IR power, peak power drops as pulse repetition rate increases, leading to reduced UV generation efficiency.
U.S. Pat. No. 5,462,433 describes a device for use in electronic warfare for delaying coherent RF signals with an adjustable time delay element. The RF signals are divided into multiple signal paths, some of the paths are delayed relative to the other paths, and then the delayed and non delayed paths are recombined to increase the pulse repetition rate.